Control circuit and control method of power converter

ABSTRACT

A control circuit of a power converter is coupled to an output stage and controls it to convert an input voltage into an output voltage and generate an output current. The control circuit includes a ripple generation circuit, a synthesis circuit, an error amplifier, a comparator and a PWM circuit. The ripple generation circuit generates a ripple signal according to an input voltage, an output voltage and output current. The synthesis circuit receives the ripple signal and a first feedback signal related to output voltage to provide a second feedback signal. The error amplifier receives the second feedback signal and a reference voltage to generate an error signal. The comparator receives a ramp signal and error signal to generate a comparison signal. The PWM circuit generates a PWM signal to control output stage according to the comparison signal. A slope of ripple signal is changed with the output current.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to power conversion; in particular, to a controlcircuit and a control method of a power converter.

2. Description of the Prior Art

In a current pulse width modulation (PWM) power converter, if a lowequivalent series resistance (ESR) capacitor is used as an outputcapacitor, ripple injection technology should be also used to achievefeedback control.

However, the current ripple injection technology can only performfeedback control through an offset of a feedback voltage. This methodwill generate an offset in a ripple signal, so that errors will occurduring an on-time of a PWM signal generated according to the ripplesignal, causing the output voltage of the power converter to drop as theload increases.

In addition, because the external ripple injection circuit usually needsto include additional passive components, the circuit cost is increased,and the internal ripple injection circuit usually needs to use adifferential circuit or an integrator circuit with a large capacitance,resulting in an excessive circuit area.

Moreover, whether it is an internal or external ripple injectioncircuit, if only a virtual ripple signal waveform is generated andinjected into the feedback path, once the load is changed, the on-timegenerated according to the virtual ripple signal waveform does not meetthe actual requirements, resulting in an offset of the output voltage.The above-mentioned problems still need to be further resolved.

SUMMARY OF THE INVENTION

Therefore, the invention provides a control circuit and a control methodof a power converter to solve the above-mentioned problems in the priorart.

An embodiment of the invention is a control circuit of a powerconverter. In this embodiment, the control circuit is coupled to anoutput stage and configured to control the output stage to convert aninput voltage into an output voltage and generate an output current. Thecontrol circuit includes a ripple generation circuit, a synthesiscircuit, an error amplifier, a comparator and a pulse width modulation(PWM) circuit. The ripple generation circuit is configured to generate aripple signal according to the input voltage, the output voltage and theoutput current. The synthesis circuit is coupled to the ripplegeneration circuit and configured to receive the ripple signal and afirst feedback signal to provide a second feedback signal, wherein thefirst feedback signal is related to the output voltage. The erroramplifier is configured to receive the second feedback signal and areference voltage to generate an error signal. The comparator isconfigured to receive a ramp signal and the error signal to generate acomparison signal. The PWM circuit is coupled to the comparator and theoutput stage and configured to generate a PWM signal according to thecomparison signal to control the output stage. A slope of the ripplesignal changes with the output current.

In an embodiment of the invention, the ripple signal includes a risingpart and a falling part, and a slope of the falling part changes withthe output current.

In an embodiment of the invention, when the output current is larger,the slope of the falling part is larger.

In an embodiment of the invention, the ripple generation circuitincludes a first current source and a second current source. The firstcurrent source is configured to generate the rising part according tothe input voltage and the output voltage. The second current source isconfigured to generate the falling part according to the output voltageand the output current.

In an embodiment of the invention, the ripple generation circuitincludes a capacitor and a switch. The switch is coupled among the firstcurrent source, the second current source and the capacitor andconfigured to selectively switch the first current source to charge thecapacitor or the second current source to discharge the capacitorcontrolled by the PWM signal.

In an embodiment of the invention, the ripple generation circuitgenerates a current signal according to a sensing signal to adjust theslope of the falling part.

In an embodiment of the invention, the control circuit includes asensing circuit. The sensing circuit is coupled to the output stage andthe ripple generation circuit respectively and configured to sense theoutput current from the output stage and provide the sensing signal tothe ripple generation circuit.

In an embodiment of the invention, the adjusted falling part equals tothe falling part before adjustment plus a current signal, and thecurrent signal is K times the sensing signal and K is a magnification.

In an embodiment of the invention, the ripple generation circuitgenerates a voltage signal according to a sensing signal to adjust theslope of the falling part.

In an embodiment of the invention, the voltage signal equals to theoutput voltage plus K times the sensing signal multiplied by aresistance value and K is a magnification.

In an embodiment of the invention, the output stage is coupled to a loadand the sensing signal is a load current flowing through the load.

Another embodiment of the invention is a control method of a powerconverter. The power converter is coupled to an output stage andcontrolling the output stage to convert an input voltage into an outputvoltage and generate an output current. The control method includessteps of: (a) generating a ripple signal according to the input voltage,the output voltage and the output current; (b) receiving the ripplesignal and a first feedback signal to provide a second feedback signal,wherein the first feedback signal is related to the output voltage; (c)generating an error signal according to the second feedback signal and areference voltage; (d) generating a comparison signal according to aramp signal and the error signal; and (e) generating a PWM signalaccording to the comparison signal to control the output stage, whereina slope of the ripple signal changes with the output current.

In an embodiment of the invention, the ripple signal includes a risingpart and a falling part, and a slope of the falling part changes withthe output current.

In an embodiment of the invention, when the output current is larger,the slope of the falling part is larger.

In an embodiment of the invention, the step (a) further includes: (a1)generating the rising part according to the input voltage and the outputvoltage; and (a2) generating the falling part according to the outputvoltage and the output current.

In an embodiment of the invention, the step (a) further includes:selectively outputting the rising part and the falling part as theripple signal according to the PWM signal.

In an embodiment of the invention, the adjusted falling part equals tothe falling part before adjustment plus a current signal, and thecurrent signal is K times a sensing signal of the output current and Kis a magnification.

In an embodiment of the invention, the control method further includes:sensing the output current and providing a sensing signal; andgenerating a voltage signal according to the sensing signal and theoutput voltage to adjust the slope of the falling part.

In an embodiment of the invention, the voltage signal equals to theoutput voltage plus K times the sensing signal multiplied by aresistance value and K is a magnification.

Compared to the prior art, the control circuit and the control method ofthe power converter of the invention have the followingadvantages/effects:

(1) there is no need to dispose passive components of the rippleinjection circuit externally, so the number of components can beeffectively reduced;

(2) since a circuit with a large capacitance (such as anintegrator/differential circuit, etc.) does not need to be disposed inthe internal ripple injection circuit, the circuit area can beeffectively reduced; and

(3) when the load increases, the drop of the output voltage can beeffectively reduced or even eliminated to achieve a stable output.

The advantage and spirit of the invention may be understood by thefollowing detailed descriptions together with the appended drawings.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 is a schematic diagram of a control circuit 1 of a powerconverter in an embodiment of the invention.

FIG. 2 is an embodiment of the ripple generation circuit 12.

FIG. 3 and FIG. 4 are different embodiments of the ripple generationcircuit 12 generating the adjusted falling part ID1 according to theoutput voltage VOUT and the sensing signal IL respectively.

FIG. 5 is a waveform timing diagram of the sensing signal IL, theadjusted falling part ID1, the rising part IU, the ripple signal VNP andthe PWM signal PWM under different load states.

FIG. 6 is a waveform timing diagram in which the slope of the ripplesignal VNP of the current power converter remains unchanged at lightload (IL=0 A) or heavy load (IL=10 A).

FIG. 7 is a waveform timing diagram of the control circuit of the powerconverter of the invention adjusting the slope of the falling part ID1of the ripple signal VNP under heavy load (IL=10 A).

FIG. 8 is a flowchart of a control method of a power converter inanother embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the invention are referenced in detail now, andexamples of the exemplary embodiments are illustrated in the drawings.Further, the same or similar reference numerals of thecomponents/components in the drawings and the detailed description ofthe invention are used on behalf of the same or similar parts.

An embodiment according to the invention is a control circuit of a powerconverter. In this embodiment, the control circuit is coupled to anoutput stage of the power converter and configured to control anoperation of the output stage to generate an output current.

Please refer to FIG. 1. FIG. 1 is a schematic diagram of the controlcircuit 1 of the power converter.

As shown in FIG. 1, the control circuit 1 is coupled to the output stageOS of the power converter and configured to control the output stage OSto convert an input voltage VIN into an output voltage VOUT and generatean output current IOUT. The control circuit 1 includes a ripplegeneration circuit 12, a synthesis circuit 14, an error amplifier 16, acomparator 18 and a PWM circuit 19.

The ripple generation circuit 12 is coupled to the synthesis circuit 14.The synthesis circuit 14 is coupled to the output stage OS, the ripplegeneration circuit 12 and the error amplifier 16 respectively. Two inputterminals of the error amplifier 16 are coupled to a reference voltageVREF and the synthesis circuit 14 respectively. Two input terminals ofthe comparator 18 are coupled to the ramp signal RAMP and the outputterminal of the error amplifier 16 respectively. The PWM circuit 19 iscoupled to the output terminal of the comparator 18 and the output stageOS respectively. The output stage OS is coupled to the input voltage VINof the power converter, the PWM circuit 19 and the synthesis circuit 14respectively. In practical applications, the control circuit 1 furtherincludes a sensing circuit 10 coupled to the output stage OS and theripple generation circuit 12, but not limited to this.

The sensing circuit 10 is used to sense the output current IOUT of theoutput stage OS and accordingly provide the sensing signal IL to theripple generation circuit 12, but not limited to this. The sensingsignal IL provided by the sensing circuit 10 is related to the outputcurrent IOUT of the power converter.

The ripple generation circuit 12 is used to generate a ripple signal VNPto the synthesis circuit 14 according to the input voltage VIN, theoutput voltage VOUT of the power converter, and the sensing signal ILrelated to the output current IOUT of the power converter. The slope ofthe ripple signal VNP will change with the output current IOUT.

In detail, the ripple signal VNP generated by the ripple generationcircuit 12 includes a rising part IU and a falling part ID, and theslope of the falling part ID changes with the output current IOUT. Forexample, when the output current IOUT is larger, the slope of thefalling part ID will be larger, and vice versa.

The synthesis circuit 14 is used to receive the ripple signal VNPgenerated by the ripple generation circuit 12 and a first feedbacksignal FB from the output stage OS, and accordingly provide a secondfeedback signal FBI to the error amplifier 16. In practicalapplications, the first feedback signal FB is related to the outputvoltage VOUT of the power converter, and the synthesis circuit 14 may bean adder for adding the ripple signal VNP and the first feedback signalFB to obtain the second feedback signal FBI, but not limited to this.

The two receiving terminals of the error amplifier 16 receive thereference voltage VREF and the second feedback signal FBI provided bythe synthesis circuit 14 respectively, and accordingly generate an errorsignal COMP to the comparator 18. The two receiving terminals of thecomparator 18 receive the ramp signal RAMP and the error signal COMPprovided by the error amplifier 16 respectively, and accordinglygenerate a comparison signal CS to the PWM circuit 19. The PWM circuit19 generates a PWM signal PWM to the output stage OS according to thecomparison signal CS to control the operation of the output stage OS.

In an embodiment, as shown in FIG. 2, the ripple generation circuit 12includes a first current source 120, a second current source 122, aswitch 124 and a capacitor C. The switch 124 is coupled among the firstcurrent source 120, the second current source 122 and the capacitor C.

The first current source 120 generates a rising part IU according to theinput voltage VIN and the output voltage VOUT of the power converter.The second current source 122 generates a falling part ID according tothe output voltage VOUT of the power converter and the sensing signalIL.

The switch 124 is controlled by the PWM signal PWM to be selectivelycoupled to the first current source 120 or the second current source122, so as to alternately switch the first current source 120 to chargethe capacitor C or the second current source 122 to discharge thecapacitor C to output the ripple signal VNP including the rising part IUand the falling part ID.

In another embodiment, as shown in FIG. 3, the ripple generation circuit12 can amplify the sensing signal IL provided by the sensing circuit 10by K times into the current signal K*IL, and adjust the slope of thefalling part ID with the current signal K*IL, so that the adjustedfalling part ID1 equals to the falling part ID before adjustment plusthe current signal K*IL, that is, ID1=ID+K*IL, where K is amagnification.

Therefore, when the sensing signal IL increases, the adjusted fallingpart ID1 will also increase (that is, the slope becomes larger) toprevent the output voltage VOUT of the power converter from offset.

For example, the ripple generation circuit 12 in FIG. 3 can include avoltage-current converter 125, an amplifier 126 and an adder 127. Thevoltage-current converter 125 and the amplifier 126 are both coupled tothe adder 127. The voltage-current converter 125 converts the outputvoltage VOUT of the power converter into the falling part ID beforeadjustment and outputs it to the adder 127. The amplifier 126 amplifiesthe sensing signal IL related to the output current IOUT by K times intothe current signal K*IL through the sensing circuit 10 and outputs it tothe adder 127. When the adder 127 receives the falling part ID beforeadjustment and the current signal K*IL, the adder 127 will add the twoto generate the adjusted falling part ID1. The output of this circuit isrelatively stable and the circuit is relatively simple.

In another embodiment, as shown in FIG. 4, the ripple generation circuit12 also generates a voltage signal VOUTA according to the sensing signalIL and the output voltage VOUT, so that the voltage signal VOUTA equalsto the output voltage VOUT plus K times the sensing signal IL multipliedby the resistance value R, that is, VOUTA=VOUT+K*IL*R, and the slope ofthe falling part ID is adjusted by the voltage signal VOUTA, and K is amagnification. This circuit control is simpler and has high noisetolerance.

Therefore, when the pumping load increases, the current signal K*ILincreases so that the voltage signal VOUTA also increases, and theadjusted falling part ID1 also increases, so as to prevent the outputvoltage VOUT of the power converter from offset.

For example, the ripple generation circuit 12 in FIG. 4 can include anamplifier 128, an adder 129 and a voltage-current converter 130. Theamplifier 128 is coupled to the adder 129. The adder 129 is coupled tothe voltage-current converter 130. The amplifier 128 amplifies thesensing signal IL provided by the sensing circuit 10 by K times into thecurrent signal K*IL and outputs it to the adder 129.

When the adder 129 receives the output voltage VOUT of the powerconverter and the current signal K*IL, the adder 129 adds the two togenerate the voltage signal VOUTA to the voltage-current converter 130.The voltage-current converter 130 converts the voltage signal VOUTA intothe adjusted falling part ID1 and then outputs it.

Please refer to FIG. 5. FIG. 5 is a waveform timing diagram of thesensing signal IL, the adjusted falling part ID1, the rising part IU,the ripple signal VNP and the PWM signal PWM under different loadstates.

As shown in FIG. 5, during the period from the time t1 to the time t3,the load state of the system is light load, and the sensing signal ILrelated to the output current IOUT is low, so that the adjusted fallingpart ID1 value is small (closer to 0 A), thus the falling slope of theripple signal VNP is also gentler.

During the period from the time t3 to the time t5, the load state of thesystem is medium load, and the value of the sensing signal IL becomeslarger, so that the adjusted falling part ID1 increases, thus thefalling slope of the ripple signal VNP increases.

During the period from the time t5 to the time t7, the load state of thesystem is heavy, and the slope value of the sensing signal IL becomeslarger, so that the adjusted falling part ID1 increases again, and thefalling slope of the ripple signal VNP becomes Steeper.

After the time t7, the load state of the system returns to light load,and the value of the sensing signal IL drops back to the original value,so that the adjusted falling part ID1 is reduced, and the falling slopeof the ripple signal VNP also becomes gentler.

Please refer to FIG. 6. FIG. 6 is a waveform timing diagram in which theslope of the ripple signal of the current power converter remainsunchanged at light load or heavy load.

It should be noted that the left half of FIG. 6 is the waveform timingdiagram of each signal when the system is under light load (the sensingsignal IL=0 A), and the right half of FIG. 6 is the waveform timingdiagram of each signal when the system heavy load (the sensing signalIL=10 A).

Under the fixed frequency buck power converter architecture, the directcurrent (DC) voltage value of the output voltage VOUT will drop duringthe load consuming period (for example, from 1.8V to 1.79V). The reasonsare as follows: when the current value of the sensing signal IL relatedto the output current IOUT increases (for example, from 0 A to 10 A),the on-time of the PWM signal PWM increases due to the fixed frequencycontrol, resulting in the increasing of the peak voltage reached by therising part IU of the ripple signal VNP, and consequently the peakvoltage reached by the second feedback signal FBI obtained by adding theripple signal VNP to the first feedback signal FB also increases.

Since the peak voltage reached by the rising part IU of the ripplesignal VNP increases during heavy load, the slope of the falling part IDof the ripple signal VNP remains the same as that at light load;therefore, it is too late for the falling part ID of the ripple signalVNP to drop to the lowest voltage value at the time TA (that is, whenthe PWM signal PWM changes from low level to high level), and a rippleoffset VD is generated.

Because when the first feedback signal FB intersects with the referencevoltage VREF, the PWM signal PWM will change from low level to highlevel. Therefore, it is necessary to satisfy the condition that thereference voltage VREF is equal to the ripple offset VD plus the firstfeedback signal FB. Since the ripple offset VD will increase with theincreasing of load consuming, the first feedback signal FB willrelatively decrease with the increase of load consuming, which causesthe output of the power converter to become unstable.

In other words, the load consuming behavior causes the end point of theripple signal VNP (or the second feedback signal FBI) to rise, which isequivalent to the rise of the reference voltage VREF, thus causing thedirect current (DC) voltage value of the output voltage VOUT of thepower converter to fall.

Please refer to FIG. 7. FIG. 7 is a waveform timing diagram of thecontrol circuit of the power converter of the invention that adjusts theslope of the falling part of the ripple signal under heavy load.

It should be noted that the left half of FIG. 7 is a waveform timingdiagram of each signal at light load (the sensing signal IL=0 A), andthe right half of FIG. 7 is a waveform timing diagram of each signal atheavy load (the sensing signal IL=10 A).

When the current value of the sensing signal IL increases (for example,from 0 A to 10 A), the on-time of the PWM signal PWM increases due tothe fixed frequency control, resulting in the increasing of the peakvoltage reached by the rising part IU of the ripple signal VNP, and thepeak voltage of the second feedback signal FBI obtained by adding theripple signal VNP to the first feedback signal FB also increases.

However, the difference between the invention and the prior art is thatthe slope of the falling part ID1 of the ripple signal VNP of theinvention under heavy load will not remain the same as the slope underlight load, but increases with the increasing of the load (that is, thecurrent value of the sensing signal IL increases), so that the adjustedfalling part ID1 can be reduced to the lowest voltage value at the timeTB (that is, the PWM signal PWM changes from low level to high level).Therefore, no ripple offset VD will be generated, and the secondfeedback signal FBI will not decrease with the increase of theconsuming, so as to achieve a stable output of the power converter.

Another embodiment according to the invention is a control method of apower converter. In this embodiment, the control method is used tocontrol an output stage of the power converter to convert an inputvoltage into an output voltage and generate an output current.

Please refer to FIG. 8. FIG. 8 is a flowchart of the control method ofthe power converter in this embodiment. As shown in FIG. 8, the controlmethod includes the following steps of:

Step S10: generating a ripple signal according to an input voltage, anoutput voltage and an output current of the power converter;

Step S12: generating a second feedback signal according to the ripplesignal and the first feedback signal, wherein the first feedback signalis related to the output voltage;

Step S14: generating an error signal according to the second feedbacksignal and the reference voltage;

Step S16: generating a comparison signal according to the ramp signaland the error signal; and

Step S18: generating a PWM signal according to the comparison signal tocontrol the output stage, and the slope of the ripple signal changeswith the output current.

It should be noted that the ripple signal generated in the step S10includes a rising part and a falling part, and a slope of the fallingpart changes with the output current. For example, when the outputcurrent is larger, the slope of the falling part is larger, and viceversa.

In an embodiment, the step S10 can generate the rising part according tothe input voltage and the output voltage and generate the falling partaccording to the output voltage and the output current, but not limitedto this.

In another embodiment, the control method further selectively outputsthe rising part and the falling part as the ripple signal according tothe PWM signal.

In another embodiment, the control method can also sense the outputcurrent from the output stage and provide a sensing signal, and generatea current signal according to the sensing signal to adjust the slope ofthe falling part. The adjusted falling part will be equal to the fallingpart before adjustment plus the current signal. The current signal is Ktimes the sensing signal, and K is a magnification, but not limited tothis.

In another embodiment, the above control method can also sense theoutput current from the output stage and provide a sensing signal, andgenerate a voltage signal according to the sensing signal and the outputvoltage to adjust the slope of the falling part. The voltage signal willbe equal to the output voltage plus K times the sensing signal, and K isa magnification, but not limited to this.

Compared to the prior art, the control circuit and the control method ofthe power converter of the invention have the followingadvantages/effects:

(1) there is no need to dispose passive components of the rippleinjection circuit externally, so the number of components can beeffectively reduced;

(2) since a circuit with a large capacitance (such as anintegrator/differential circuit, etc.) does not need to be disposed inthe internal ripple injection circuit, the circuit area can beeffectively reduced; and

(3) when the load increases, the drop of the output voltage can beeffectively reduced or even eliminated to achieve a stable output.

What is claimed is:
 1. A control circuit of a power converter, thecontrol circuit being coupled to an output stage and configured tocontrol the output stage to convert an input voltage into an outputvoltage and generate an output current, and the control circuitcomprising: a ripple generation circuit, configured to generate a ripplesignal according to the input voltage, the output voltage and the outputcurrent; a synthesis circuit, coupled to the ripple generation circuitand configured to receive the ripple signal and a first feedback signalto provide a second feedback signal, wherein the first feedback signalis related to the output voltage; an error amplifier, configured toreceive the second feedback signal and a reference voltage to generatean error signal; a comparator, configured to receive a ramp signal andthe error signal to generate a comparison signal; and a pulse widthmodulation (PWM) circuit, coupled to the comparator and the output stageand configured to generate a PWM signal according to the comparisonsignal to control the output stage, wherein a slope of the ripple signalchanges with the output current.
 2. The control circuit of claim 1,wherein the ripple signal comprises a rising part and a falling part,and a slope of the falling part changes with the output current.
 3. Thecontrol circuit of claim 2, wherein when the output current is larger,the slope of the falling part is larger.
 4. The control circuit of claim2, wherein the ripple generation circuit comprises: a first currentsource, configured to generate the rising part according to the inputvoltage and the output voltage; and a second current source, configuredto generate the falling part according to the output voltage and theoutput current.
 5. The control circuit of claim 4, wherein the ripplegeneration circuit comprises: a capacitor; and a switch, coupled amongthe first current source, the second current source and the capacitor,configured to selectively switch the first current source to charge thecapacitor or the second current source to discharge the capacitorcontrolled by the PWM signal.
 6. The control circuit of claim 2, whereinthe ripple generation circuit generates a current signal according to asensing signal to adjust the slope of the falling part.
 7. The controlcircuit of claim 6, further comprising: a sensing circuit, coupled tothe output stage and the ripple generation circuit respectively andconfigured to sense the output current from the output stage and providethe sensing signal to the ripple generation circuit.
 8. The controlcircuit of claim 6, wherein the adjusted falling part equals to thefalling part before adjustment plus the current signal, and the currentsignal is K times the sensing signal and K is a magnification.
 9. Thecontrol circuit of claim 2, wherein the ripple generation circuitgenerates a voltage signal according to a sensing signal to adjust theslope of the falling part.
 10. The control circuit of claim 9, whereinthe voltage signal equals to the output voltage plus K times the sensingsignal multiplied by a resistance value and K is a magnification. 11.The control circuit of claim 9, wherein the output stage is coupled to ad and the sensing signal is a load current flowing through the load. 12.A control method of a power converter, the power converter being coupledto an output stage and controlling the output stage to convert an inputvoltage into an output voltage and generate an output current, thecontrol method comprising steps of: (a) generating a ripple signalaccording to the input voltage, the output voltage and the outputcurrent; (b) receiving the ripple signal and a first feedback signal toprovide a second feedback signal, wherein the first feedback signal isrelated to the output voltage; (c) generating an error signal accordingto the second feedback signal and a reference voltage; (d) generating acomparison signal according to a ramp signal and the error signal; and(e) generating a PWM signal according to the comparison signal tocontrol the output stage, wherein a slope of the ripple signal changeswith the output current.
 13. The control method of claim 12, wherein theripple signal comprises a rising part and a falling part, and a slope ofthe falling part changes with the output current.
 14. The control methodof claim 13, wherein when the output current is larger, the slope of thefalling part is larger.
 15. The control method of claim 13, wherein thestep (a) further comprises: (a1) generating the rising part according tothe input voltage and the output voltage; and (a2) generating thefalling part according to the output voltage and the output current. 16.The control method of claim 13, wherein the step (a) further comprises:selectively outputting the rising part and the falling part as theripple signal according to the PWM signal.
 17. The control method ofclaim 13, wherein the adjusted falling part equals to the falling partbefore adjustment plus a current signal, and the current signal is Ktimes a sensing signal of the output current and K is a magnification.18. The control method of claim 13, wherein the step (a) furthercomprises: sensing the output current and providing a sensing signal;and generating a voltage signal according to the sensing signal and theoutput voltage to adjust the slope of the falling part.
 19. The controlmethod of claim 18, wherein the voltage signal equals to the outputvoltage plus K times the sensing signal multiplied by a resistance valueand K is a magnification.